Composite thin films containing cellulose nanocrystal (cellN) polyanions embedded between either poly(diallyldimethylammonium chloride) (PDDA) or chitosan were fabricated using the layer by layer (LBL) deposition technique The in plane and out of plane elastic constants of the composites were measured using Brillouin light scattering as a function of film thickness and cellulose content Compared to the pure cast polymer films the addition of celIN raises the elastic constants within the growth plane by a factor of 2 and 3 for [chitosan/cellN] and [PDDA/cellN] films respectively, while in the growth direction the elastic constant increases by 50% for [PDDA/cellN] and not at all for [chitosan/cellN] With increasing amounts of celIN in the films the stiffness Increases in the growth plane at a higher rate than in the growth direction These trends reflect the contribution of the cellulose nanocrystals within and cross layers to load transmission The results are interpreted in terms of processes that occur during film deposition and the resulting spatial arrangements of the nanocrystals